Dinitrile compounds, such as adiponitrile (ADN), are important commercial chemicals. The most important application of dinitriles being as intermediates in the production of diamine monomers, which are useful in the synthesis of various polyamide polymers. The hydrogenation of ADN provides hexamethylenediamine (HMD), which is one of the essential ingredients used to manufacture nylon 66 (N66). N66 is produced by reacting HMD with adipic acid (AA) to form an aqueous salt solution. This salt solution is then heated to remove the solvent water, and subsequently to react the amine end of the HMD with the acid end of the AA to produce an amide linkage and a molecule of water. This reaction is known as a condensation polymerization because it produces a molecule of water for each amide linkage formed. The formation of multiple amide linkages leads to the formation of the N66 polymer. The N66 polymer can be used to produce synthetic fibers and engineering polymers which are of great commercial value.
In order to produce high quality polyamides, the essential ingredients must be of extremely high purity. The presence of impurities causes weakening of the fiber and undesirable color of the polymer. Undesirable impurities in the diamines are removed primarily by distillation operations, while impurities in dicarboxylic acids such as AA are removed primarily by crystallization. Some of the impurities in the diamines are produced during the hydrogenation of dinitriles, and would exist even if the dinitriles were absolutely pure. Other impurities in the diamines result from the presence of impurities that are contained in the dinitriles, in particular impurities that are formed by unwanted side reactions of dinitriles during dinitriles production and refining. One of the most detrimental impurities to N66 quality is 2-aminomethylcyclopentylamine (AMC), which is formed primarily by the hydrogenation of the impurity 2-cyanocyclopentylideneimine (CPI) that is formed via the cyclization of ADN.
The removal of CPI from ADN is difficult, as the relative volatility of CPI to ADN is 1.45. However, the removal of AMC from HMD is much more difficult, as the relative volatility of AMC to HMD is 1.20. Thus, to maintain low levels of AMC in the HMD product, it is best to remove the CPI from the ADN prior to hydrogenation.
The presence of CPI in ADN has been a major problem from the first days of ADN manufacture. While there have been several processes for making ADN, they all have had to deal with the CPI problem. The primary method for removing CPI from ADN is vacuum distillation, but when sufficiently low levels of CPI cannot be achieved, other options have been employed to remove the CPI after distillation. U.S. Pat. No. 3,496,212 describes the use of a water soluble aldehyde in combination with an extraction step using an aromatic solvent and water followed by an additional distillation step. Canadian patent 672,712 employs ozone treatment of the ADN to destroy the CPI. U.S. Pat. No. 3,758,545 uses paraformaldehyde to chemically react with the CPI. U.S. Pat. No. 3,775,258 hydrolyzes the CPI to the ketone using an acidic catalyst at 140° C. Canadian patent 1,043,813 uses a weak cation exchange resin to remove the CPI from the ADN. All of these processes are directed to the removal of CPI from the ADN. A far better approach would be to prevent the formation of the CPI during the ADN manufacturing process.
One of the leading commercial process for producing ADN is the hydrocyanation of 1,3-butadiene to 3-pentenenitrile (3PN), followed by the hydrocyanation of the 3PN to ADN. These hydrocyanations are performed using a nickel (0) catalyst stabilized with phosphite ligands. These phosphite ligands can be either monodentate or bidentate. The hydrocyanation of 3PN also requires the use of a Lewis acid co-catalyst. When using a bidentate ligand catalyst system, zinc chloride (ZnCl2) is a suitable Lewis acid. One of the advantages of the direct hydrocyanation process is the hydrocyanation of 3PN takes place at mild temperature conditions where virtually no CPI is generated. However, in order to refine the crude ADN to the high purity product required for hydrogenation to HMD, several distillation steps are required. Because of the very low vapor pressure of ADN, these distillations involve temperatures as high as 200° C. At these high temperatures CPI generation occurs during these distillation operations.
There is therefore a need in the art for processes for the preparation of dintriles, such as ADN, of high purity in which the formation of byproducts such as CPI is suppressed. It has now been identified that the addition of a Brønsted acid to dinitriles suppresses side reactions of the dinitriles which can lead to the formation of unwanted by products, such as the formation of CPI from ADN.